Oil: It’s the lifeblood of your classic’s engine, and one of the best proactive vehicle maintenance procedures. Clean, healthy oil is vital to ensure proper lubrication of your engine’s moving parts. It also provides cooling, and prevents wear on parts such as the pistons, cylinder walls, valvetrain, crankshaft, bearings, and rods. Change it regularly and you’ll likely be rewarded with an engine that is smooth running and long lasting. We know of a great example of such dedicated maintenance: an original owner of a 1970 Buick Wildcat convertible has logged over 500,000 miles on the car without a major engine rebuild.

And if you don’t perform regular oil changes, you can expect lengthy repairs and a sizeable hole in your wallet. Many cars we come across for sale in the classifieds need engine work; they are often the result of neglected maintenance and infrequent oil changes. Problems such as valve sealing, worn piston rings, excessive oil consumption, and blow-by are a direct result of this–even cars that display relatively low miles on the odometer can be suffering from these problems, simply as a result of a lack of oil changes. And when it comes to older cars (the kind most of us like to work on), choosing the right oil and changing it regularly is one of the most critical aspects in keeping it running at its best. It should also be one of the first things on your to-do list, especially if you’ve recently bought a car, as in the case of our subject, a 1975 Cadillac Eldorado coupe powered by an enormous 500 cubic-inch (8.2-liter) V-8. Follow with us now as we outline the correct procedure for changing the oil on this big beast.

Special thanks to Rob Neely and Scott Watkins of Neely’s Auto Service for their help with this article, along with Betty Royce of Royce Classic Auto Service.

Our subject vehicle was recently purchased from an older gent. Originally a rust belt car, it had spent a considerable part of its latter years in Florida before being brought back North. The big Caddy hadn’t been regularly driven for a few years prior to the current owner purchasing it, so an oil change and inspection were a top priority in order to get it back on the road.

Choosing the correct oil is crucial to ensuring optimal engine performance. Older V-8 Detroit cars like our Eldorado sometimes run a touch warm and have fairly substantial bottom end clearances, so in order to prevent overheating and provide adequate lubing, thicker-grade oils are the order of the day. We elected to refill the Caddy with Pennzoil 10W30 mineral oil. According to the company’s website, Pennzoil conventional mineral oils are claimed to clean out up to 15 percent of sludge in an engine’s oil passages on the first oil change, and are especially developed for start/stop driving where heat and friction are at their worst. Considering that our Caddy hadn’t been driven any distance in a while, this oil seemed a good choice.

Before starting your oil change, it’s recommended that you get all the tools you need for the job. With our Caddy now inside a service bay, we’ve got our fresh oil, a sample filter wrench, axle stands, hydraulic jack, and funnel for pouring the oil in the crankcase. As is the case with many 1970s and older cars, we had to order our oil filter, so it had yet to arrive when this picture was taken.

If you’re going to change the oil in your driveway/shop, it’s absolutely crucial to use a good quality jack and axle stands. You also need to make sure they’re rated for the weight of the vehicle. Considering that our Caddy weighs almost 5,200 pounds (over 2.5 tons), a heavy-duty jack and stands are essential. Therefore, you need to use a jack and axle stands that are at least rated for 3-ton capacity. Most regular, store-purchased 2-ton jacks can’t handle a car of this size and weight. And your body can’t handle a car’s size or weight if a flimsy jack fails!

Although for illustration purposes we’ll be using a hoist, you can see our jack and axle stands in place. Never use the jack on its own to support the weight of the car, and make sure you use jacking points recommended by the manufacturer, such as frame rails or lower control arms. If you don’t, you can damage the underside of the car, including the floor pans and fuel and brake lines.

Here’s our big Caddy secured on the hoist, ready for its oil change. A popular technique is getting the car–and oil–up to operating temperature first to make sure it all gets drained out.

Although your car probably only has one plug, these huge 500 cubic-inch Caddy V-8s feature a deep sump with two oil drain plugs. Both need to be removed during the oil change procedure to ensure as much of the old lube as possible drains out.

Before you begin, you also need to make sure you have a suitable device for catching the old oil. If you can get one, a purpose designed catch can and oil storage container is recommended, as it will save time. If you can’t, go to your local auto parts store and use a plastic drain. Once the old oil is out, you can pour the contents from the drain into an empty container. Many auto parts stores now allow you to dispose of your old lube, so take it there once you’re finished with the oil change.

With the catch can in place, we’re ready to remove the oil drain plugs. You need to check the size of the plug heads before beginning; the wrong-size socket can damage the plugs. Our 1975 Eldorado hails from the pre-metric era, so all the bolts and nuts on this car are in English measurements. The drain plugs are 9/16 in diameter, so we used the correct-size socket.

Now that both drain plugs have been removed, the old oil drains out. Dirty oil should look black, but ours was “milkshake” contaminated by antifreeze, ultimately traced to a leaking intake manifold. It was good we caught this during our oil change, as it could have caused major problems down the road.

Here you can see the old oil drain into the container.

With the old oil drained, it’s time to reinstall the drain plugs. In order to prevent stripping the threads, hand tighten them for the first few turns before using a socket. Check the torque specs to ensure the plugs are nice and snug without being too tight.

For removing the filter, it’s best to use a wrench. Shown are two different types, one with crescent ends, and the other a socket designed to grip the base of the filter and remove it slowly using a wrench. Other types, like band and claw/socket tools, work fine too.

On many older cars, filters are generally more accessible compared with modern ones, which makes socket wrenches highly suitable as they prevent damaging the filter when installing and removing it. If you’ve got several different classics that require different size oil filters, it’s a good idea to have an oil filter socket set like this one.

Here, a filter wrench is used to loosen the old filter, part No. 85045.

After a few twists with the socket, we put it aside and remove the filter by hand. Don’t get splashed!

Choosing the right oil filter is just as crucial as the correct lube. Most traditional filters use cellulite to trap foreign matter and sludge. However, the effectiveness of cellulite tends to drop off fairly rapidly and even at its most efficient, these types of filters are only able to trap about 80 percent of dirt and sludge. A number of modern filters use synthetic fibers blended with cellulite, which provide much better trapping efficiency, extending filter life and minimizing engine wear. Even so, you should still change your filter every time you swap your oil. Also make sure when shopping, you choose a filter with the correct part number for your car. Compare our replacement 500 ci V-8 filter (right) to a modern four-cylinder filter (left).

Before installing the new filter on our Caddy, we half fill it with new Pennzoil 10W30 to prevent dry bearings during the post-change startup. In order to provide adequate sealing and prevent the gasket material from drying out, it’s essential to lube the gasket surface of the filter with a light coating of the new oil.

With that done, we can install the new filter and slowly tighten it until it seals. Your car probably requires tightening the filter a certain amount with a filter wrench after it’s hand-tight, but don’t over-tighten it.

Because oil changes can be messy, it’s a good idea to clean and wipe any excess from the underside of the car. That way if there are any oil spots in the driveway later on, you’ll know they aren’t the result of an oil change.

Most pre-1980 cars also require greasing the front end as part of an oil change. For this procedure, make sure you have access to an automotive grease gun like this.

Front end grease nipples on big 1970s GM cars are found on the upper and lower spindles and steering links. All of them need to be re-greased every 3,000 miles to prevent premature wear on the steering components. Greasing them takes a matter of minutes.

Now that we’ve replaced the filter, greased the front end, and checked that the drain plugs have been properly tightened, we can lower the car, pop the hood, and pour in the new oil. The first task is removing the oil filler cap on the passenger-side valve cover.

Here we pour fresh Pennzoil 10W30 into the engine via the passenger side valve cover. These big V-8s require almost 6 quarts of oil to fill them.

Once in, some of the oil will make its way to the filter. So in order to make sure it’s at the correct level, you’ll need to start the engine and run it for a few seconds until the oil light on the dash goes out.

Pull out the dipstick…

…And check the level. Make sure the oil on the dipstick is well within the safe range and as close to the full mark as possible. Don’t overfill the engine, as it can put excessive strain on the oil seals, which will eventually cause multiple leaks.

While you’re changing your oil, it’s a good time to inspect the air filter. If it’s clogged replace it, as a dirty filter can reduce power output and increase fuel consumption.

Another good idea is to check the belts and hoses for any cracks or fraying, as rubber deteriorates over time. Replacing them now is cheap insurance, especially since a blown hose or fractured belt can cause overheating and possible engine failure.

With the oil, lube, and filter done, the front end greased, and the belts, hoses, and tire pressure checked, our big Caddy is getting closer to hitting the road again.